The present invention relates to the shielding of electromagnetic radiation in order to minimize electromagnetic coupling, and to the prevention of electrostatic discharge. More specifically, the present invention provides improved shielding and grounding of the openings in shielded equipment cages, e.g., in computer equipment, telecommunications equipment, and the like.
Two problems that have long plagued electrical equipment designers are electromagnetic coupling (EMC) and electrostatic discharge (ESD). EMC is the unintentional transfer of electromagnetic radiation from one or more electrical components to another electrical component. EMC produces undesirable noise in and/or interferes with the normal operation of the receiving electrical component. EMC can occur any time an electrical component is located within an electromagnetic radiation rich environment, such as proximate other electrical components. To prevent EMC, a system of electrical components, e.g., the various interconnected circuit boards of a computer, is often contained within a metal cage, e.g., a processor cage, that blocks out, i.e., "shields" the system from most electromagnetic radiation existing outside the metal cage, and that likewise prevents electromagnetic radiation produced within the cage from affecting equipment external to the cage.
ESD is the discharge of static electrical charge that occurs when two objects having different static charge states, e.g., different amounts of charge, opposite polarity charge, etc., are closely proximate. Because ESD can result in large, although short duration, voltages which can interfere with the operation of or damage electrical devices, ESD must be avoided whenever possible. To prevent static charge buildup that can cause ESD, the cage, electrical components therewithin, and any connections thereto share the same ground, i.e., are commonly grounded. For instance, a computer may have a processor cage shielding the computer's main circuit boards, and a frame surrounding and supporting a hard drive, power supply, the processor cage, etc. To prevent ESD between the frame and processor cage, the frame and processor cage should be commonly grounded whenever a connection is made therebetween.
While a properly grounded cage may protect electrical circuitry within the cage from EMC and ESD, often the electrical circuitry within the cage must connect to external circuitry/equipment. To allow for such connections, openings are provided in the cage. These openings form an EMC path into the cage, and if not properly grounded, form a conduit or "situs" for ESD.
One approach for reducing EMC and ESD through a shielded cage opening is to plug the opening with a shielded plug. For instance, U.S. Pat. No. 5,600,092 to Patscheck et al. ("the '092 Patent") shows a single contact spring that removably fills an opening of a shielded cage when no cables connect to or through the cage opening. The '092 Patent, however, does not address EMC shielding or ESD protection when the contact spring is removed from the cage opening, such as when a cable extends therethrough. EMC protection is required both when the external connection is present and when it is absent, and continuous grounding is needed to continuously prevent ESD.
Another approach for reducing EMC and ESD through an opening in a shielded cage is to commonly shield, i.e., within a single cage, the opening as well as any external electrical components coupled via the opening, see, for example, U.S. Pat. No. 5,652,410 to Hobbs et al. However, for large external components, e.g., computers, printers, etc., shielding is often impractical and does not prevent EMC between the caged components and the commonly shielded components. That is, EMC protection is provided only from radiation sources external to both the cage and the commonly shielded electrical components.
Yet another shielding method mounts a shield having a central aperture such as those manufactured by Instrument Shielding Specialties within an opening. In order to hold the shield securely in place and thus to avoid the inconsistent shielding caused by shield movement, central aperture type shields are often adhesively mounted or mounted mechanically via screws or the like. Shield mounting thereby becomes time consuming, slows equipment assembly and teardown, and is unacceptable for many applications.
Accordingly, a need exists for a method and apparatus for shielding cage openings whether or not the openings are in use, without requiring the shielding of equipment or components external to the cage. The shield must be mechanically stable to ensure a continuous grounding and shielding, and must be designed to facilitate assembly and teardown.